Carbon dioxide (CO2) is the primary greenhouse gas emitted through human activities. In 2013, CO2 accounted for about 82% of all U.S. greenhouse gas emissions from human activities. CO2 is naturally present in the atmosphere as part of the Earth's carbon cycle (the natural circulation of carbon among the atmosphere, oceans, soil, plants, and animals). Human activities are altering the carbon cycle—both by adding more CO2 to the atmosphere and by influencing the ability of natural sinks, like forests, to remove CO2 from the atmosphere. While CO2 emissions come from a variety of natural sources, human-related emissions are responsible for the increase that has occurred in the atmosphere since the industrial revolution.
The main human activity that emits CO2 is the combustion of fossil fuels (coal, natural gas, and oil) for energy and transportation, although certain industrial processes and land-use changes also emit CO2. The main sources of CO2 emissions in the United States Carbon dioxide (CO2) emissions in the United States increased by about 7% between 1990 and 2013. Since the combustion of fossil fuel is the largest source of greenhouse gas emissions in the United States, changes in emissions from fossil fuel combustion have historically been the dominant factor affecting total U.S. emission trends. Changes in CO2 emissions from fossil fuel combustion are influenced by many long-term and short-term factors, including population growth, economic growth, changing energy prices, new technologies, changing behavior, and seasonal temperatures. Between 1990 and 2013, the increase in CO2 emissions corresponded with increased energy use by an expanding economy and population, and an overall growth in emissions from electricity generation. Transportation emissions also contributed to the 7% increase, largely due to an increase in miles traveled by motor vehicles.
Airborne CO2 has come under stricter emission regulations. Reducing greenhouse gases will soon become mandatory. Separation of CO2 from power plants and other commercial plants that generate large amounts of CO2 is likely imminent. The cost of CO2 capture is generally estimated to represent three-fourths of the total cost of a carbon capture, storage, transport, and sequestration.
A CO2 removal technology that can be retrofitted into existing power plants or applied to existing storage facilities to reduce its volume is needed. A low cost CO2 removal process is also needed.